The present invention is related to the field of tuners for use in television receivers and the like including a mixer for combining RF carriers and corresponding local oscillator signals generated by a voltage controlled oscillator to produce an IF signal.
Many types of mixer circuits including a bipolar transistor as the mixing device are known. However, recently mixer circuits including a field effect transistor (FET) as the mixing device have been proposed because an FET exhibits a more nearly square law transfer characteristic than a bipolar transistor. As a result, an FET mixer tends to generate fewer undesirable cross-modulation and intermodulation products than a bipolar transistor mixer.
Unfortunately, although an FET mixer exhibits a more nearly square law transfer characteristic than a bipolar mixer, an FET mixer tends to have a lower conversion gain than a bipolar transistor. The conversion gain of an FET mixer is dependent not only on the bias conditions of the FET, but also on the amplitudes of the local oscillator signals coupled to it. Unfortunately, when a voltage controlled oscillator utilizing a varactor diode is utilized in conjunction with an FET mixer, it is not generally possible to ensure the desired local oscillator signal amplitude for an acceptable conversion gain throughout the entire frequency range of the local oscillator signals since the local oscillator signals tend to exhibit relatively large amplitude variations throughout their frequency range.
To remedy this undesirable situation, it has been proposed to vary the DC operating point of an FET mixer in response to the tuning voltage which normally controls the frequency of the local oscillator signals generated by the voltage controlled oscillator associated with the FET mixer and tends to track their amplitude variations to compensate for the amplitude variations of the local oscillator signals (see, for example, U.S. Pat. No. 3,976,944). Unfortunately, this approach is only a compromise solution since, as earlier noted, the conversion gain of an FET mixer is not only dependent on the bias point of the mixer but also the amplitudes of the local oscillator signals coupled to it.